Recently, as a method of braking an electric railway car which moves along a track line, a regenerative braking method, that is, a method of withdrawing electric energy from the kinetic energy of an electric railway car again when the accelerated electric railway car reduces speed in order to stop, has been used in order to save on energy.
Such a regenerative braking method has advantages in that problems, such as noise attributable to mechanical braking and the abrasion of a brake shoe, can be prevented as well as that the power consumption of the overall railway system can be reduced, so that the using range of the regenerative braking method is on an increasing trend.
However, with regard to such a regenerative braking method, when an accelerated railway car reduces its speed in order to stop using the regenerative braking method while in motion, an electric motor operates as a power generator and performs dynamic braking, so that a large amount of regenerative power is instantly generated. Such regenerative power instantly applies a large amount of voltage to a wiring, with the result that the voltage of the wiring varies, so that the system is unstable. Further, when an electric railway car which is going backwards cannot accommodate the voltage, the regenerative power only functions as a factor in the change in the voltage of the wiring, thereby causing trouble in the electric railway car which is going backwards.
Meanwhile, generally, a method of supplying voltage to an electric railway car is performed using a method of converting Alternating Current (AC) voltage into DC voltage using a rectifier and then supplying the DC voltage. The circuit of the rectifier is connected in a forward diode manner, so that the regenerative power of an electric railway car is not returned to power source side.
Therefore, surplus regenerative power which was not consumed on the wiring is converted into heat energy and then consumed, increases the voltage of the wiring and automatically blocked due to the over voltage of an entering electric railway car, or cannot be accommodated by the wiring, so that the regenerative power is consumed by a magnetic resistor, thereby causing regeneration-cancelled status.
In order to solve such a problem, an inverter for regeneration is installed in a DC rectifier so that the voltage of a wiring is converted into AC and then the resulting AC is returned to a power source side, or a separate storage device for receiving regenerative power from the wiring and storing the received regenerative power is installed. However, when regenerative power generated by a number of railway cars is returned to a power source side via an inverter without being filtered, the regenerative power including harmonics may unexpectedly damage receptors.
Further, Korean Patent Registration No. 0659366 entitled “Regenerative Power Storage System of Metro Railway”, applied for by the present applicant and then registered, may be provided as an example which uses a separate storage device.
Korean Patent Registration No. 0659366 includes wiring configured to function as an electricity supply path for at least one electric railway car and, at the same time, function as a path used to return electricity generated by regenerative braking when the electric railway car reduces speed; a gate electrically connected to the wiring and capable of controlling the input/output of power; an energy storage device connected to the gate, configured to convert supplied electric energy into kinetic energy when power is supplied, configured to store the kinetic energy, configured to convert the stored kinetic energy back again into electric energy in response to a control signal, and configured to output the resulting electric energy; a voltage detection unit electrically connected to the wiring, and configured to detect the voltage of the wiring, a gate driving circuit configured to drive the gate in response to the control signal based on information detected by the voltage detection unit; and a microprocessor configured to determine the voltage detected by the voltage detection unit, configured to output the control signal to the gate driving circuit, and configured to control the operation of the energy storage device, thereby controlling the conversion between electric energy and kinetic energy and controlling the input/output of power.
However, the energy storage device is configured in mechanical manner using a fly-wheel. Accordingly, there are problems in that a lot of noise is produced during operation, the volume of the energy storage device itself is large, and a process of converting electric energy into kinetic energy is included, so that it is inefficient to receive and store instant electric energy.
Further, the system is configured to detect the electricity status of wiring while it is installed on the ground, so that there is a problem in that all the running electric railway cars may be affected when trouble occurs on the system and the system malfunctions or is out of control.